[python] fixed sklearn test on python 2.7 (#888)
[python] fixed sklearn test on python 2.7 (#888)

* fixed sklearn test on python 2.7

* commit to show that problem has been solved

* come back to python 3.6

* removed warnings check

microsoft/LightGBM,Allardvm/LightGBM,henry0312/LightGBM,olofer/LightGBM,wxchan/LightGBM,microsoft/LightGBM,wxchan/LightGBM,Allardvm/LightGBM,henry0312/LightGBM,wxchan/LightGBM,olofer/LightGBM,wxchan/LightGBM,olofer/LightGBM,Laurae2/LightGBM,Laurae2/LightGBM,henry0312/LightGBM,microsoft/LightGBM,microsoft/LightGBM,wxchan/LightGBM,microsoft/LightGBM,Laurae2/LightGBM,Allardvm/LightGBM,olofer/LightGBM,olofer/LightGBM,henry0312/LightGBM,henry0312/LightGBM,Allardvm/LightGBM,Laurae2/LightGBM,Allardvm/LightGBM,Laurae2/LightGBM